Injections of various substances into avian eggs have been employed to decrease post-hatch mortality rates, increase the potential growth rates or eventual size of the resulting chicken, and even to influence the gender determination of the embryo. Similarly, injections of antigens into live eggs have been employed to incubate various substances used in vaccines which have human or animal medicinal or diagnostic applications.
Examples of substances which have been proposed as viable treatment (or harvestable vaccine material) alternatives for delivery via in ovo injection of avian embryos include live culture vaccines, antibiotics, vitamins, and even competitive exclusion media (a live replicating organism). Specific examples of treatment substances are described in U.S. Pat. No. 4,458,630 to Sharma et al, and U.S. Pat. No. 5,028,421 to Fredericksen et al.
Conventionally, the physical injection has been typically targeted at preferred positions within the egg in order to administer the substance into specific developing regions of the embryo. As understood by those of skill in the art, as the incubation period progresses towards maturity (i.e., hatching), the embryo and its membranes, e.g., the air cell, the allantois, and yolk sac, correspondingly change in both volume and position within the egg shell. Additionally, the quantitative volume of the enclosed fluids vary as well; for example, the density of the allantois (fluid, solid) varies as a function of time over the incubation period.
Thus, selection of both the site and time of treatment can impact the effectiveness of the injected substance as well as the mortality rate of the treated embryos. See e.g., U.S. Pat. No. 4,458,630 to Sharma et al., U.S. Pat. No. 4,681,063 to Hebrank, and U.S. Pat. No. 5,158,038 to Sheeks et al.